In the textile and allied fields where elastomeric properties are desired in certain products and end uses, rubber has long been used. For many applications, natural or synthetic rubber falls short because of undesirable aesthetics or insufficient physical properties. In many instances it must be covered with other materials, natural or synthetic, for use in garments. Therefore much effort has gone into the development of materials which have the desirable elastic properties of rubber but not the undesirable aesthetic or physical properties mentioned above. Much of this work has evolved around the incorporation of long-chain "soft segments" into polymers made up of varying proportions of more easily crystallized thermoplastic "hard segments", resulting in segmented copolymers which are thermoplastic, elastomeric, and capable of being extruded into filaments, fibers or other desired shapes.
Charch and Shivers reported the use of polyester ether condensation block copolymers ("Part II; Elastomeric Condensation Block Copolymers", Charch and Shivers, Textile Research Journal, 1959, p. 536.) which are capable of being shaped into fibers, their compositions being based primarily on the use of poly (oxyethylene oxyterephthalogyl) as the hard segment and poly (oxyethylene) as the soft segment. The low glass transition temperature, resultant extensibility, reactive difunctionality and economy explain this soft segment's extensive use in various other studies; however, its susceptability to room temperature recrystallization at high molecular weights, even as part of a block copolymer, adversely effects elastic properties and limits useful molecular weights to less than 1500.
In constructing segmented copolymers which have sufficiently rapid hardening rates to allow satisfactory handling after extrusion, it has been found necessary to have certain percentages of specific dicarboxylic acids in the hard and soft segments and certain percentages of specific low molecular weight diols in the hard segment. For instance it is reported essential in U.S. Pat. No. 3,763,109 that at least about 70 mole % of the dicarboxylic acid in the polymer be terephthalic acid and at least about 70 mole % of the low molecular weight diol in the polymer be 1,4-butane diol. Also, at least about 70 mole % of the low molecular weight diol in the hard segment must be 1,4-butane diol. Further the sum of the percentages of the carboxylic acid in the hard and soft segments which is not terephthalic acid and the low molecular weight diol in the hard segment which is not 1,4-butane diol can not exceed about 30 mole %. Copolyesters having fewer 1,4-butylene terephthalate units than assured by the foregoing requirements do not have sufficiently rapid hardening rates for satisfactory handling after extrusion.